Mantle Flow and Determining Position of LAB Assuming Isostasy

Leszek Czechowski

doi:10.1007/s00024-019-02093-8

Abstract

The assumption of isostatic equilibrium is often used in geophysics; e.g., the depth of the lithosphere–asthenosphere boundary (LAB) is determined using data regarding the gravity field and topography, together with the assumption of isostasy. However, isostasy implies a hydrostatic state, which is contrary to the mantle convection hypothesis, which states that most of the mantle matter is in motion. We therefore discuss herein the question of when the assumption of isostasy can be used. It is suggested that isostasy may be used for parts of oceanic plates (except for subduction zones, hotspots, and oceanic ridges) and for many continental regions (except for postglacial regions and regions of intensive volcanic or tectonic activity). Moreover, using the results of a numerical model of convection and calculations of dynamic topography, it is shown that some generalization of isostasy is possible in the form of deep dynamic isostasy (DDI). It is also indicated that, for some regions without isostatic equilibrium (e.g., postglacial regions), it is possible to use the “isostatic” method with some corrections to the results.

Highlights

Lithosphere, asthenosphere, and the lithosphere– asthenosphere boundary (LAB) are important terms in the physics of the solid Earth, but their meanings are understood differently by different specialists (e.g., Jones et al 2010; Grad et al 2014; Czechowski and Grad 2015, 2018)
Methods based on Eqs. (1–2) may seem like a silver bullet for many geophysical problems. That such methods suffer from some drawbacks: 1. For local isostatic equilibrium, the lithosphere should behave like independent, separated columns that float on a liquid layer
Compared with ‘‘classic’’ isostasy, the difference lies in the position of the compensation level, which for deep dynamic isostasy (DDI) is located at the bottom of the convective cell instead of the asthenosphere

Summary

Introduction

Lithosphere, asthenosphere, and the lithosphere– asthenosphere boundary (LAB) are important terms in the physics of the solid Earth, but their meanings are understood differently by different specialists (e.g., Jones et al 2010; Grad et al 2014; Czechowski and Grad 2015, 2018). The asthenosphere is a layer with lower effective viscosity (compared with the underlying mantle), separating the lithospheric plates from the Isostasy is a state of gravitational equilibrium such that blocks of the lithosphere ‘‘float’’ in the asthenosphere at an elevation that depends on their thickness and density according to Archimedes’s law. When a certain part of the Earth’s crust reaches the state of isostasy, it is said to be in isostatic equilibrium. Some regions are not in isostatic equilibrium, e.g., postglacial regions which are still uplifting (like Fennoscandia) after melting of the ice cover

Objectives

Methods

Conclusion